Fretting vs Pitting Corrosion – What’s the Difference?

Corrosion is one of the most prevalent causes of damage to industrial equipment and machinery. This process can lead to material degradation, system failure, and even catastrophic accidents. While many types of corrosion can occur, fretting and pitting corrosion are the most common. In this blog, we’ll explore the differences between these two types of corrosion and how they can be managed effectively.

What is Fretting Corrosion?

Fretting corrosion occurs when two surfaces come into contact and experience small relative movements in the presence of moisture and a corrosive environment. This constant friction can cause wear, leading to pitting or cracking of the material. Fretting corrosion can also reduce the effective surface area of components, potentially leading to failure. This type of corrosion is particularly common in bolted joints, where the tightening and loosening of the bolt can cause fretting.

Fretting removes material through oxidation caused by repeated contact between two surfaces. This minor contact, or fretting, creates friction and heat, which erode the surface leaving tiny pits called “fret marks”. Fretting can occur when surfaces are subject to cycling loads, vibrations and relative motion due to assembly misalignment or component deformation. Over time wear debris accumulates, lowering the coefficient of friction and further reducing performance until failure occurs. Properly designed components (geometry, material selection) with adequate lubrication and cleaning will greatly reduce fretting-related failures in machines operating under static or dynamic conditions.

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What is Pitting Corrosion?

Pitting corrosion occurs when small pits or holes form on the surface of a material due to chemical or electrochemical reactions. This type of corrosion can occur on any area of a metal surface, but it most commonly occurs in areas of high stress or where crevices retain moisture. Pitting corrosion can reduce material thickness, stress concentration, and system failure.

Pitting corrosion is a special localized corrosion characterized by creating small holes or pits on the metal surface. It usually occurs in chloride-rich environments, such as coastal areas, and chemical reactions with oxygen, chlorides, sulfates, or other corrosive agents can cause it. This form of corrosion can lead to significant loss of structural integrity if left unchecked and untreated. It’s important to recognize pitting corrosion early so necessary maintenance and repairs can be done promptly before a catastrophic failure occurs.

What is the difference between Fretting and Pitting Corrosion?

While fretting and pitting corrosion are both forms of material degradation caused by corrosion, their characteristics and occurrences differ. As mentioned earlier, fretting corrosion occurs due to the movement of two surfaces against each other, while pitting corrosion results from chemical or electrochemical reactions. Fretting corrosion typically occurs in bolted joints or other areas where metal-to-metal contact occurs, while pitting corrosion can occur on any part of a metal surface. Fretting corrosion can lead to stress concentration, while pitting corrosion can cause material thinning.

How Can Fretting and Pitting Corrosion Be Prevented?

Preventing corrosion is critical to maintaining equipment and machinery in good working order. To prevent fretting corrosion, designers can use materials with a low coefficient of friction, such as polymers or coatings. Oil or grease lubricants can also be applied to reduce the amount of metal-to-metal contact. Designers can use materials resistant to the chemical or electrochemical reactions that cause pitting to prevent pitting corrosion. Using coatings or painting can also protect surfaces from corrosive environments.

Conclusion:

Both fretting and pitting corrosion can cause significant damage to equipment and machinery if not managed effectively. Understanding the differences between these two types of corrosion can help identify potential problems and enable engineers to take appropriate actions to prevent them. Ongoing maintenance and equipment monitoring is essential to prevent corrosion from developing and catch it early when it does. By implementing effective preventive measures, companies can ensure the reliability and safety of their equipment and minimize the costs associated with repairs and downtime.

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